In 2004, an estimated 19 million Americans were current illicit drug users. Importantly, racial and ethnic minority populations continue to be deeply impacted by concomitant drug use, HIV/AIDS, and other infectious diseases. Moreover, there is concern that data from predominantly white male populations may not be indicative of HIV disease in women and/or racial/ethnic minorities. Therefore, the HIV Epidemiologic Research (HER) Study was established in 1993 to prospectively define the biological, psychological, and social effects of HIV infection on the health of US women. Because the HER Study enrolled a large number of IVDUs, the overall prevalence of hepatitis C virus (HCV) was 56.5%, thus providing the unique opportunity to compare HCV mono-infection and HCV/HIV co-infection within the sample population over time. Although the precise mechanism(s) remain unclear, it is clear that HIV co-infection leads to increased HCV replication as HCV RNA levels are significantly elevated in HCV/HIV co-infected persons compared to HCV mono-infected persons. While hepatocytes are the major site of infection, there is evidence for extrahepatic replication of HCV in peripheral blood mononuclear cells (PBMCs). Several studies have suggested that HCV replication within PBMCs may allow for viral reactivation and viral rebound upon removal of HCV treatment. Nonetheless, the role that immunosuppression may play in boosting extrahepatic replication of HCV has not been well defined. Moreover, HCV evolution and compartmentalization may contribute to the chronicity of infection, liver disease progression, and HCV treatment outcome, although these phenomena have not been adequately addressed in the context of HCV/HIV co-infection or in a longitudinal manner. Thus, a new investigator with clinical and laboratory experience in both HCV and HIV is proposing this R21. The goals of this application are to define important qualitative differences in HCV mono-infection versus HCV/HIV co-infection, as measured by extrahepatic replication and evolutionary dynamics of HCV, among female IVDUs using a novel strand-specific real-time PCR assay and a rigorous bioinformatics- phylogenetics approach. This proposal will provide important data on co-factors of virus transmission and identify potential markers of HCV disease progression and treatment failure. [unreadable] [unreadable] [unreadable]